SEVASTOPOL, Crimea — Russian forces and their Crimean militia allies were reported on Thursday to have released the commander of the Ukrainian Navy, seized in his own headquarters here as Moscow’s annexation of the strategic Black Sea peninsula forced the authorities in Kiev to begin planning for the evacuation of their forces to mainland Ukraine.

But, against a backdrop of European threats to ratchet up sanctions against Russia, the limits of the likely Western response to the Kremlin’s actions emerged in a sharp relief on Thursday when President Obama ruled out military action in the region.

“We are not going to be getting into a military excursion in Ukraine,” Mr. Obama told Mark Mullen, an anchor at NBC 7 in San Diego. “What we are going to do is mobilize all of our diplomatic resources to make sure that we’ve got a strong international correlation that sends a clear message.”

He was speaking hours before European leaders were to meet in Brussels to debate further measures after Washington and European leaders ordered a preliminary array of assets freezes and visa restrictions on officials involved in the takeover of Crimea.

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Ukrainian air defense forces on Wednesday moved their belongings out of dormitories near a blockaded military base in the village of Lubimovka, Crimea.Credit
Uriel Sinai for The New York Times

Mr. Obama said the United States would continue to “ratchet up” pressure on the Russians by the use of further sanctions, but said that “I think even the Ukrainians would acknowledge that for us to engage Russia militarily would not be appropriate and wouldn’t be good for Ukraine either.”

After a short debate on Thursday, the lower house of Parliament in Moscow voted overwhelmingly, as expected, to approve a treaty of annexation signed by President Vladimir V. Putin on Tuesday. Only a single vote was cast against, by Ilya Ponomaryov, one of few independent legislators. The margin showed both the extent of the Kremlin’s control of the legislature and the breadth of approval for the takeover of Crimea.

The upper house of Parliament was expected to follow suit on Friday, brushing aside Western threats.

In a speech to the German Parliament in Berlin on Thursday, Chancellor Angela Merkel warned Russia that the European Union summit would signal readiness to impose further economic sanctions — a sensitive political issue for some Europeans fearful of Moscow’s response. But she made the threat conditional on further moves by the Kremlin, saying that stepped-up reprisals could be imposed at any time “if there is a worsening of the situation.”

Many European states, notably Germany, have close economic ties to Russia, a major energy supplier.

In Crimea, the detention of the Ukrainian naval commander, Adm. Serhiy Haiduk, reflected the blistering pace of the Kremlin’s takeover of Crimea. Mr. Putin has described Crimea as an “inalienable” part of Russia, a statement reinforced by Moscow’s military occupation of key facilities.

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Russia Takes Crimean Naval Base

Ukrainian and Russian officials spoke in Sevastopol after militiamen, backed by Russian forces, seized the headquarters of the Ukrainian Navy and raised a Russian flag.

Thousands of Ukrainian soldiers and sailors have been trapped on bases and other installations here for more than two weeks, surrounded by heavily armed Russian forces and loosely organized local militia.

On Wednesday, militiamen backed by Russian forces seized the headquarters of the Ukrainian Navy in Sevastopol and detained Admiral Haiduk in what appeared to be the start of a concerted effort to oust the Ukrainian armed forces from bases throughout the peninsula.

Word of the admiral’s release on Thursday came first from the website of Ukraine’s acting president, which did not provide details.

The Ukrainian government said on Wednesday that it had drawn up plans to evacuate its roughly 25,000 military personnel and their families to mainland Ukraine.

While the provisional government in Kiev has insisted that Russia’s annexation of Crimea is illegal and has appealed to international supporters for help, the evacuation announcement by the head of the national security council, Andriy Parubiy, effectively amounted to a surrender of Crimea, at least from a military standpoint.

In the fevered atmosphere following Russia’s takeover, a Ukrainian diplomat said further military moves by the Kremlin were possible. “There are indications that Russia is on its way to unleash a full-blown military intervention in Ukraine’s east and south,” Yuri Klymenko, Ukraine’s ambassador to the United Nations in Geneva, told diplomats on Thursday at a briefing on human rights in Ukraine, Reuters reported.

In Kiev, the provisional government said on Wednesday that it would quit the Commonwealth of Independent States, the group of former Soviet republics, and that it was considering imposing visa requirements on Russian citizens — a step that would potentially create huge inconveniences for Ukrainians as well in the likely event that Russia reciprocated.

But the Ukrainian authorities seemed to be backing away from the threat on Thursday. Arseniy P. Yatsenyuk, the interim prime minister, was quoted as saying that Ukraine should not be in a hurry to impose visa restrictions that were “most unlikely” to sway Russia and could rebound on Ukrainian citizens in the east of the country.

The United Nations said that Secretary General Ban Ki-moon was to travel to Moscow and Kiev on Thursday and Friday for meetings with leaders, including Mr. Putin, whose moves to reclaim Crimea have set off the biggest crisis in East-West relations since the Soviet Union’s demise more than two decades ago.

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Russian flags waved from a watch tower after Russian forces seized the Ukrainian naval headquarters in Sevastopol.Credit
Uriel Sinai for The New York Times

Mr. Ban has expressed disappointment over the Kremlin-backed referendum in Crimea last weekend that created the basis for Russia’s annexation, but he has said nothing about whether he considered the Russian step to be illegal. The United States and other Western members of the Security Council proposed a resolution on Saturday declaring the referendum illegal, but Russia, acting alone, vetoed that measure.

At the Ukrainian naval headquarters here on Wednesday, soldiers with machine guns, wearing green camouflage but no identifying insignia, were deployed in and around the base. A large military truck just outside the base bore the black-and-white license plates of the Russian forces.

Although the gates were forced open during the initial storming of the base, there were no reports of shooting or injuries. And while there was no indication that the Ukrainian government was prepared to issue a formal surrender in Crimea, capitulation by military units surrounded throughout the peninsula seemed increasingly inevitable.

When asked why they did not return fire, one Ukrainian soldier leaving the base said, “We had no order and no weapons.” Another said, “We met them empty-handed.”

Governments scrambled for a response to the fast-moving events.

Speaking in Britain’s Parliament on Wednesday, Prime Minister David Cameron said the world’s leading industrialized countries should consider ejecting Russia from the G-8. The United States, Britain and their allies in the older Group of 7 will meet in The Hague next week to debate further measures.

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Pro-Russia militia members on Wednesday forced their way through an entrance to the Ukrainian naval headquarters in Sevastopol, Crimea. They later raised the Russian flag in the square nearby.Credit
Andrew Lubimov/Associated Press

Before the crisis in Crimea, Mr. Putin was scheduled to host a gathering of the G-8 countries in June in Sochi, where the Winter Olympics were recently held, but Western countries have suspended their participation. “If we turn away from this crisis and don’t act,” Mr. Cameron said, “we will pay a very high price in the longer term.”

Germany’s government has expressed caution, reflecting its intertwined economic relations with Russia. Although Chancellor Merkel took a tough tone with Moscow in public last week, business executives are reluctant to jeopardize trade ties, and diplomats and officials steeped in decades of conciliation with Russia are hesitant to sever avenues for negotiation. High-level talks scheduled for April have not been canceled.

The German government spokesman, Steffen Seibert, said on Wednesday that Russia was “pursuing a path of international isolation, and it is a path containing great dangers for the coexistence of states in Europe.”

He also gave the first official response to Mr. Putin’s appeal on Tuesday to the German people to support what he depicted as Russian reunification, just as Russia supported German reunification in 1990.

German reunification brought together two German states, Mr. Seibert said, while “Russia’s intervention, by contrast, is leading to a division of Ukraine.”

David M. Herszenhorn reported from Sevastopol, Crimea; Andrew E. Kramer from Kiev, Ukraine; and Alan Cowell from London. Reporting was contributed by Michael D. Shear from Washington; Mark Landler from Vilnius, Lithuania; Melissa Eddy and Alison Smale from Berlin; Rick Gladstone from New York; Somini Sengupta from the United Nations; Noah Sneider from Sevastopol; and Patrick Reevell from Simferopol, Crimea.